Echo is a problem related to the perceived speech quality in telephony systems with long delays, e.g. telephony over long distances or telephony systems using long processing delays, like digital cellular systems. The echo arises in the four-to-two wire conversion in the PSTN/subscriber interface. To remove this echo, echo cancellers are usually provided in transit exchanges for long distance traffic, and in mobile services switching centers for cellular applications.
Due to the location of the echo canceller it is made adaptive; the same echo canceller is used for many different subscribers in the PSTN. This adaption is necessary not only between different calls, but also during each call, due to the non-fixed nature of the transmission network, e.g. phase slips, three-party calls, etc.
A problem with this filter adaption process is that the filter may diverge if the input signal decreases to a level that approaches the background noise level. To prevent the filter from diverging in situations where the background noise level is comparable to the signal level, it has been suggested in WO 93/09608 to inhibit updating of the filter when the power of the input signal is less than a given threshold. To overcome problems related to using a fixed threshold, the threshold is made adaptive in WO 93/09608. The method of in WO 93/09608 is based on comparison of the power of the input signal and the background noise level. The adaption is prevented if the power of the echo (power of input signal--echo path attenuation ERL) is less than the background noise level plus a margin of 1 to 5 dB.
A problem with the described approach is its dependence on an accurate estimate of the echo path attenuation ERL. If a large value of ERL is estimated, the adaption may be completely inhibited. Hence, the filter coefficients are frozen, and, assuming ERL is estimated from the filter coefficients, no new estimate of ERL will be found. If the characteristics of the echo generating system now change, the filter will not be able to adapt to the new situation. Thus, the method suggested in WO 93/09608 is too conservative, i.e. filter updating is inhibited also in situations where this should be avoided.